Our recent findings have generated a new paradigm for nonmutagenic mammary carcinogenesis. We demonstrated that the activating Hras1 mutations found in NMU-induced tumors arise as background mutations within cells of the developing gland, and that NMU enhanced the phenotypic penetrance of these mutations by initiating alterations in DNA conformation. We further identified a cell-type-specific, DNA structure within the Hras1 promoter of normal rat mammary cells(RMCs) in vivo. The structure has characteristics of H-form DNA and includes a putative ets transcription factor binding site. Our results further demonstrated that depending upon hormonal status of the animals, RMCs can switch between states where the structural feature is present or absent from the Hras1 promoter. For this reason, we call this DNA structure a Conformation Toggle Switch(CS). In the Fischer 344(F344) and Sprague-Dawley(SD) strains, which are sensitive to mammary carcinogenesis, a carcinogenic dose of NMU initiated the loss of this structure from the Hras1 promoter of RMCs. While the Hras1 CS was restored in normal RMCs by 120 days after exposure, its loss was irreversible in cells that gave rise to tumors. By contrast, the same exposure to induce CS disruption in Copenhagen(Cop) rats, which are highly resistant to mammary carcinogenesis. NMU also failed to promote the outgrowth of pre-existing Hras1 mutants present in the mammary epithelium of resistant Cop rats. Moreover, NMU-induced disruption of the CS in sensitive rats was inhibited by a diet supplemented with a chemopreventive dose of high selenium garlic. Together our results suggest that NMU-induced alterations in DNA conformation promote the outgrowth of pre-existing mutants by irreversibly deregulating expression of Hras1 and other ets-responsive genes, thereby increasing the phenotypic penetrance of the conditional Hras1 oncomutations. Phenotypic analysis of resistant(F344 khi Cop)F1 progeny further indicated that the suppression of CS disruption was medicated by one or more dominant suppressors expressed in RMCs of Cop rats. Together our findings indicated that these genetic suppressors of CS disruption are putative tumor suppressor genes. The overall goal of this proposal is to use genetic linkage analysis to map this novel suppressor(s) of mammary carcinogenesis on the rat genome in preparation for positional cloning and application to studies of human breast cancer risk, early detection and prevention.